Recent studies of the sensory neocortex of mammals have shown that the anatomical and functional parcelling of the cortex into distinct regions is heavily influenced by afferent axons origination in the thalamus. These findings as well as advances in anterograde and retrograde tracing methods have led to a reexamination of thalamocortical afferent terminations and their relationship to cortical laminar differentiation. In contrast to the visual and somatosensory systems, little is known concerning the morphology and distribution of specific classes of medial geniculate afferents TO auditory cortex. Recent novel findings in my laboratory using sensitive neuroanatomical tracers have revealed a patch- like organization of thalamocortical afferents arising from the ventral division of the medial geniculate body (vMGB). The afferent patches have an intermittent distribution resembling the patchy distribution of binaural interaction classes defined in physiological studies. In the tangential plane, the patches form elongated bands exceeding 2.0 mm in length parallel to isofrequency contours. These results are consistent with model of vMGB organization containing functionally distinct parallel pathways to lamina III/IV of AI. However, morphological evidence for such pathways is lacking. In Aim 1, experiments are proposed to determine the relationship between recently discovered thalamocortical patches and binaural interaction columns in normal rabbits. The auditory cortex will be physiologically mapped with multiple tangential electrode penetrations after the ipsilateral MGB has received an injection of the anterograde tracer biocytin. Coronal and tangential sections through the mapped cortex will processed to reveal the anterograde labeled patches and the physiologically determined binaural interaction columns allowing direct comparison of the two. In Aim 2, axonal classes arising from separate subdivisions of the MGB will be characterized on the basis of laminar termination and spatial organization following 3-dimensional reconstruction from serial sections. In Aim 3, the anterograde transport of biocytin will be combined with fluorescent immunocytochemistry to determine the laminar projection of MGB neurons expressing different calcium binding proteins. In Aim 4, the developmental time-course for the arrival, in growth and arborization of thalamocortical axons will be compared with the laminar differentiation of the auditory cortex. The postnatal remodeling of thalamocortical arbors and their temporal relationship with dendritic resorption by target cells within lamina III and IV will be examined. These studies will provide new information on the thalamocortical circuits subserving tonotopic and binaural maps and the morphological substrates for parallel MGB pathways to AI. They will also provide a morphological basis for understanding the trophic role of auditory afferents in cortical development and dendritic growth.